[NDX2] Maximum size USB and what type of USB possible?

What will be the maximum capacity of the USB-stick in the NDX2 ?

And what type (best sort) USB can used in the NDX2? Will that be USB 3.0 or 3.1 ?

For now i guess that the (high res) music files on a USB-stick can be used for superb music quality.

You can use a 2 terabyte portable ssd drive via the usb connection, these are usb 3.1. They are quite expensive though!

You can use pretty much any USB storage with a standard Type A USB lead. I’m not convinced that you need anything fancy to cope with the relatively low data rates of audio files. Naim say the max recommended capacity is 100,000 tracks.
Use the Server input to browse if you want anything more than a very basic folder view.

From Atom/Nova point of view: I use a compact external SSD, with USB-3. (Designed for portability/cost; not for speed.)
The streamer itself won’t care about the speed actually; the benefit of faster interface is when filling the disk/SSD from a PC/Mac/whatever. I.e my drive “only” does 150MByte/s, but for delta-copying new files, that’s good enough. It being a SSD makes the handling and checking for “delta” pretty fast.
The other benefit of the SSD (compared to HDD) is, it’s completely silent when plugged in. Maybe the SSD helps with faster initial indexing by the streamer, never compared that.

If you get a fancy USB-C connector with 3.1/3.2+ speed (more than 5Gbps original USB3 speed), the gains will be small and seldom (when copy really large data to it); and you’ll need a USB-A-to-C cable with it. It may be easier to grab an older device with matching cables and “sufficient” speed.

Server input, that will be the USB on the backside of my NDX2? (the only one on the back)

You can use either front or rear USB input. Both should work via the app USB input or Server input.